Hydrophobicity denotes a chemical property representing interaction between water and an object, i.e. conceptually having no affinity to water.
FIG. 13 shows a contact angle between water and the surface of an object. A higher contact angle represents higher hydrophobic tendency. For example, a contact angle of 180 degrees means that the object surface is treated to be super-hydrophobic. In this case, the water remains in the form of a perfect sphere on the object surface.
In general, hydrophobic materials are easily found in nature. Taro leaves or lotus leaves are typical hydrophobic materials. It has been found out by Wenzel's and Cassie's that the hydrophobicity on the leaves' surface is due to micro-porous structures on the surface. FIG. 14 shows a plant leaf having hydrophobicity and its porous structure.
A chemical or physical process may be used for treating the surface of an object to be hydrophobic or super-hydrophobic.
FIG. 15 shows an example of the surface structure of an object modified through a physical process. In order to provide super-hydrophobicity, carbon nano-tubes are used to form a porous structure on a smooth surface of the object. If a water drop is fallen onto the surface of the carbon nano-tubes, it remains in a nearly spherical form.
The chemical process involves applying fluoric coating and the like on the surface of an object, e.g. when manufacturing a fry pan. That is, the chemical composition of an object surface is changed to allow the surface to be hydrophobic or super-hydrophobic. Fluorocarbon polymers exhibit a strong hydrophobic tendency, in particular, among other chemical materials.
U.S. Pat. Nos. 4,869,922, 6,649,222 and 5,733,610 disclose chemical processes of treating the surface of an object to be hydrophobic or super-hydrophobic.
U.S. Pat. No. 4,869,922 discloses a surface treating process of coating polyfluorocarbon on the surface of an object using vacuum plasma to exhibit hydrophobicity. In this patent, a mixture of hydrogen gas and monomer C-F gas is injected into a discharge space under a pressure of 1 Torr. In addition, a 27.12 MHz RF (radio frequency) power of 40 to 80 W is applied for 5 to 20 minutes to coat polyfluorocarbon on the surface of an aluminum specimen with a size of 20×20×1 mm, so that the specimen surface can be modified to be hydrophobic.
The U.S. Pat. No. 6,649,222 discloses a process of treating the surface of a specimen to be super-hydrophobic using modulated glow discharge plasma. A 13.56 MHz modulated frequency power of 50 to 75 W is applied for 20 to 90 minutes under a pressure of 300 to 400 mTorr, and a monomer C-F gas is used to treat the surface of a non-metallic specimen, such as PE, PP, silicon, glass and PET, with an area of 2 to 20 cm2.
U.S. Pat. No. 5,733,610 discloses a surface treatment process of providing hydrophobicity under atmospheric pressure. A frequency of 3000 Hz is used to treat the surface of organic and silicon wafer specimen under atmospheric pressure.
The conventional techniques using a vacuum system for treating the surface to be hydrophobic or super-hydrophobic are carried out only in a closed system. Thus, the conventional techniques cannot implement a continuous or automated process in which the specimen is moved and simultaneously treated, and thus, there is a problem in that the techniques cannot be applied to mass-production industries.
Further, expensive vacuum equipment for use in a vacuum system and its relevant maintenance lead to significant cost increases.
In addition, since polymers change their properties at high temperature, they should be processed at lower temperatures within a few seconds. Thus, there is another problem in that it is difficult to control process conditions for fine treatment.
Furthermore, there is a further problem in that a process of treating the surface of an object to be hydrophobic under atmospheric pressure should employ a batch system to suppress arc generation.